1. Field of the Invention
The present invention relates to a television receiver and particularly to a picture-display-region discriminating circuit of a so-called wide-screen television receiver for discriminating whether or not any picture is displayed on a specific region where a picture or a letter is to be displayed.
2. Description of the Related Art
There have been increased a video software obtained by converting a movie having a rectangular picture (also referred to as a wide screen) called a Vista Vision size or Cinema Scope size into a picture with an aspect ratio of 4:3 to be displayed on a screen of a television receiver of a standard system (NTSC system, PAL system, SECAM system and so on), and a broadcasting program having a picture obtained by converting a broadcasting signal of a high-definition television system used for a television receiver of a wide screen corresponding to a frame size of an aspect ratio of 16:9 into a signal used for a television receiver of the standard television system.
There is available on a market a television receiver called a wide-screen television receiver having a display screen with an aspect ratio of 16:9 so as to display pictures of these video softwares for the wide screen on the screen with using 525 scanning lines similarly to that of the standard television system.
This wide-screen television receiver is provided with various display modes used for a viewer to watch such video software or broadcasting program on the television receiver with an optimum frame size of such video software or broadcasting program. The viewer operates a remote controller or the like to select a preferable display mode.
For example, the wide-screen television receiver provides the following display modes.
(1) The first display mode is used when a rectangular picture of a video software is displayed on the screen of the wide-screen television receiver; the displayed picture is enlarged in the frame-height direction with reference to a center thereof in accordance with a ratio of a frame height of a standard frame size to that of a wide frame size, thereby a zoom picture being enlarged so as to occupy a substantially entire wide screen and displayed thereon.
(2) The second display mode is used when a letter-box picture of a video software whose picture display region is different depending upon a video software and a picture of a foreign movie with a superimposed dialogue is displayed on the screen of the wide-screen television receiver; since it is impossible to fix a reference point of an enlargement of the displayed picture in the frame-height direction, the picture is allowed to scroll in the frame-height direction until only an upper or lower end portion scrolls off the screen.
(3) The third display mode is used when a picture of the standard system with an aspect ratio of 4:3 is displayed on the wide screen so as to occupy an entire screen in the frame-height direction; in this case, portions where no picture is displayed are produced on both of left and right ends of the wide screen, which prevents the wide screen from being utilized effectively. Accordingly, in this case, a central portion of the picture is not processed but both of left and right end portions thereof are enlarged in the frame-width direction, thereby a rectangular picture being displayed on the entire wide screen.
However, it is not easy for the viewer to select an optimum display mode for the video software or broadcasting program.
A system for an automatic selection of the display mode is considered, wherein a broadcasting side or a software production side inserts an identification information indicative of a kind of a software into a vertical blanking period of a video signal and a television receiver automatically switches a display mode based on the identification information. However, realization of the system for the automatic selection of the display mode encounters problems, e.g., requirement of preparation on all the broadcasting sides or software production sides and manufacturing costs of a television receiver of such system. Therefore, the system will not be put into practice in the near future.
In order to solve the above problems, after studying distribution of pictures on the screen of the television receiver or the like, the same assignee has already developed a low-cost television receiver for solving the above problems and disclosed it in Japanese patent application (filed on Aug. 16, 1994). According to this application, as shown in FIG. 1, stripe picture detection regions a, b are respectively set on an upper portion and a center portion of the picture screen, and small picture detection regions c, d, e, f, are set on a lower portion of the picture screen. It is detected whether or not any picture is displayed on each of the above six picture detection regions a to f. Based on the detection result, it is discriminated which kind of the picture the video software has. Based on the discriminated result, an optimum display mode for the video software is selected.
However, such picture-display-region discriminating system of the above television receiver encounters the following problems.
(1) Only six picture detection regions are provided on the picture screen. The number of the picture detection regions is not enough. If the video softwares having many kinds of picture sizes are distributed in the future, then it will be impossible with flexibility to discriminate all kinds of the video softwares with using only the above six picture detection regions. Moreover, for example, if a level of the video signal corresponding to a center portion of a picture screen is changed considerably, then the discrimination may be erroneous.
(2) Since the picture-display-region discriminating system employs only one reference black level, the discrimination result is presented as one of two values obtained from discriminating whether or not the level of the video signal corresponding to each of the picture detection region is equal to or smaller than the black level. As a result, if the black level itself is changed, it may lead to erroneous discrimination.
(3) Since the reference level (black level) is fixed, if the black level is changed to a higher level, then a portion where no picture is displayed is erroneously discriminated as a portion where any picture is displayed.
Accordingly, the above picture-display-region discriminating system encounters the problems to be solved for precisely discriminating whether or not any picture is displayed on each of the picture detection regions a to f.